I had a similar problem (at least from what i can tell from your description, kinda sketchy). Im definetly no guru, but you should definetly reset your needles to factory settings before droppin some coin on anything. I know, it might not make sense, but it solves 80% (or so it seems) of the problems the peeps bring up on here.

In my experience (needles are a kryptonite for me) even mildly incorrect needle settings can make your engine do some wacky things which are mind boggling and have in the past really stumped me as to a solution.

Edit:
Otherwise try swapping glow plugs.
Seriously though, the info so far is like asking "theoretically, if -such and such- happens, what is my problem?"
So answer, what engine are you working with? What is your fuel nitro percentage? etc. etc.

Edit 2:
Try starting it without the air filter (just for a few seconds, its really not good, but it can provide some answers)